Many industrial waters tend to be corrosive. Such waters, when in contact with a variety of metal surfaces such as ferrous metals, aluminum, copper and its alloys, tend to corrode one or more of such metals or alloys.
Ferrous metals such as carbon steel are among the most commonly used structural materials in industrial systems. It is generally known that in industrial systems having a ferrous metal in contact with an aqueous solution, corrosion (both general and localized corrosion) of the metal is one of the major problems. Loss of the metals from surfaces resulting from general corrosion causes deterioration of the structural integrity of the system or structure because of reduction of mechanical strength. It can also cause problems such as underdeposit corrosion, increased heat transfer resistance, or even blockage of the flow lines in other parts of the system due to the transport and accumulation of corrosion products in areas with low flow rates or geometric limitations. Localized corrosion (e.g., pitting ) may pose an even greater threat to the normal operation of the system than general corrosion because such corrosion will occur intensely in one particular location and may cause perforations in the system structure carrying an industrial water stream. Obviously, these perforations may cause leaks which require shutdown of the entire industrial system so that repair can be made. Indeed, corrosion problems usually result in immense maintenance costs, as well as costs incurred as a result of equipment failure. Therefore, the inhibition of metal corrosion in industrial water is critical.
Corrosion protection of ferrous metals in industrial water systems is often achieved by adding a corrosion inhibitor. Many corrosion inhibitors, including chromate, molybdate, zinc, nitrite, orthophosphate, and polyphosphate have been used previously, alone or in combination, in various chemical treatment formulations. However, these inorganic chemicals are either toxic and detrimental to the environment, or are not very effective against localized corrosion, especially at economically feasible and/or environmentally acceptable low dosage levels, although they can usually provide satisfactory protection against general corrosion (e.g., corrosion rate .ltoreq.3 mpy). Some organic phosphonates, such as 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and aminotrimethylenephosphonic acid (AMP) have been used previously as corrosion inhibitors, alone or in combination with other corrosion inhibitors, in various chemical treatment formulations. However, the effectiveness of these phosphonate based treatments is generally significantly lower than the treatments based on inorganic inhibitors.
U.S. Pat. No. 5,167,866 discloses that certain phosphonomethyl amine oxides can be used as scale and corrosion inhibitors in aqueous systems. In subsequent publications [D. Hartwick, J. Chalut and V. Jovancicevic, Corrosion/93, paper no. 266, NACE, (1993); D. Hartwick, J. Richardson, V. Jovancicevic and M. Peters, Corrosion/94, paper no. 517, NACE, (1994)], ethanolamine bisphosphonomethyl N-oxide (EBO) was further identified to be a particularly effective pitting inhibitor. Nevertheless, the concentrations needed to obtain sufficient inhibition still appear to be prohibitively high (e.g., &gt;50mg/l EBO is needed to obtain an anodic inhibition efficiency of greater than 40%).
Scale build-up is another serious problem in industrial water systems. The build-up of deposit (scales) interferes with heat transfer, e.g., from the inside surface of a heat exchanger tube (i.e.- the process side) to the cooling medium such as water. The reduction of the rate of heat transfer occurs because the scales formed generally have a lower heat transfer coefficient than the metal tube itself. Thus, scaling reduces the efficiency of the system. Further, scaling and deposits can lead to corrosion underneath the deposits on the metallic surface and reduce the useful life of the equipment. Calcium carbonate or sulfate as well as iron oxides and hydroxides generated in the corrosion process are some of the most commonly observed scale formers in industrial water systems.
The utilization of water which contains certain inorganic impurities, and the production and processing of crude oil/water mixtures containing such impurities, is plagued by the precipitation of these impurities with subsequent scale formation. In the case of water which contains these contaminants, the harmful effects of scale formation are generally confined to the reduction of the capacity or bore of receptacles and conduits employed to store and convey the contaminated water. In the case of conduits, the impedance of flow is an obvious consequence. However, a number of equally consequential problems are realized in specific utilizations of contaminated water. For example, scale formed upon the surfaces of storage vessels and conveying lines for process water may break loose and these large masses of deposit can be entrained in and conveyed by the process water to damage and clog equipment through which the water is passed, e.g., tubes, valves, filters and screens. In addition, these crystalline deposits may appear in, and detract from, the final product which is derived from the process, e.g., paper formed from an aqueous suspension of pulp. Furthermore, when the contaminated water is involved in a heat exchange process, as either the "hot" or "cold" medium, scale will be formed upon the heat exchange surfaces which are contacted by the water. Such scale formation forms an insulating or thermal pacifying barrier which impairs heat transfer efficiency as well as impeding flow through the system.
Most industrial waters contain alkaline earth metal cations, such as calcium, barium, magnesium, etc. and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction products, a solid phase of calcium carbonate will form. Calcium carbonate is the most common form of scale.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of the water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes and harbors bacteria. This scale is an expensive problem in many industrial water systems causing delays and shutdowns for cleaning and removal.
Scale deposits are generated and extended principally by means of crystal growth; and various approaches to reducing scale development have accordingly included inhibition of crystal growth, modification of crystal growth and dispersion of the scale-forming minerals.
While calcium sulfate and calcium carbonate are primary contributors to scale formation, other salts of alkaline-earth metals and the aluminum silicates are also offenders, e.g., magnesium carbonate, barium sulfate, and the aluminum silicates provided by silts of the bentonitic, illitic, and kaolinitic types among others.
Numerous compounds have been added to these industrial waters in an attempt to prevent or reduce scale and corrosion, such as low molecular weight poly(acrylic acid) polymers. Another class of compounds are the well known organophosphonates which are illustrated by the compounds hydroxyethylidene diphosphonic acid (HEDP) and phosphonobutane tricarboxylic acid (PBTC). Another group of active scale and corrosion inhibitors are the monosodium phosphinico(bis) succinic acids which are described in U.S. Pat. No. 4,088,678.
Many organophosphorus compounds have been disclosed as scale inhibitors. For example, N,N-bis (phosphonomethyl)-2-amino-1-propanol and derivatives are disclosed in U.S. Pat. No. 5,259,974; ether diphosphonates are disclosed in U.S. Pat. No. 5,772,893; N-substituted aminoalkane-1,1-diphosphonic acids are disclosed in U.S. Pat. No. 3,957,160; and propane-1,3-disphosphonic acids are disclosed in U.S. Pat. No. 4,246,103. Further, N-bis(phosphonomethyl) amino acids for the prevention of calcium carbonate scale are disclosed in U.S. Pat. Nos. 5,414,112 and 5,478,476. 1,1-diphosphonic acid compounds are disclosed in U.S. Pat. Nos. 3,617,576 and 4,892,679.
Hydroxyimino alkylene phosphonic acids are disclosed in U.S. Pat. No. 5,788,857. Furthermore, there are several references to the use of N,N-bis-phosphonomethyl N-oxides such as the ethoxylated N,N-bis-phosphonomethyl 2-(hydroxy)ethylamine N-oxides in U.S. Pat. No. 4,973,744; N,N-bis(phosphonomethyl)-2-amino-1-propanol N-oxide in U.S. Pat. No. 5,259,974; oxidized tertiary amines in U.S. Pat. Nos. 5,096,595 and 5,167,866; N,N-bis-phosphonomethyl taurine N-oxide in U.S. Pat. No. 5,051,532; and tetrakis(dihydrogen phosphonomethyl)ethylene diamine N,N-dioxides in U.S. Pat. No. 3,470,243. However, these compounds are structurally different from those 1,1-diphosphonic acid N-oxides described herein, in that these compounds are 1,3-diphosphonic acid 2-N-oxides, while the compounds of the instant invention are 1,1-diphosphonic acid-1-N-oxides. As will be seen from examples which follow, these structural differences lead to superior anti-scale and anti-corrosion characteristics.
Apparently, there is a need for a corrosion inhibitor that can effectively prevent both general corrosion and localized (e.g., pitting) corrosion of ferrous metals and can also efficiently prevent scale formation on metallic surfaces in contact with the waters of various systems, such as industrial process waters.
Among the objectives of this invention are: to provide a family of N-alkylene-1,1-diphosphonic acid-1-N-oxides that can effectively provide inhibition of localized (pitting) corrosion of ferrous metals in contact with such systems; to provide a family of N-alkylene-1,1-diphosphonic acid-1-N-oxides that can efficiently reduce general corrosion of ferrous metals in contact with such systems; to provide a family of N-alkylene-1,1-diphosphonic acid-1-N-oxides that can efficiently prevent scale formation on metallic surfaces in contact with such systems; to provide a family of N-alkylene-1,1-diphosphonic acid-1-N-oxides that can simultaneously prevent localized corrosion, general corrosion of ferrous metals, and scale formation on metallic surfaces in such systems, and to provide a family of N-alkylene-1,1-diphosphonic acid-1-N-oxides which are biocide stable and calcium tolerant.